Here I want to show a 'step by step' sequence (based on a Fiat Twin-Cam 1592cc cylinder head) to demonstrate how the inlet flow (taking the thing as a whole from manifold entry to combustion chamber) can be increased substantially by careful port, valve and valve seat modification. The technique of test, modify, develop using a flowbench is common to ALL heads, not just Fiat ones! Power and torque will be measurably enhanced if we can improve the flow into the cylinder during the valve event, and although it will not be a direct proportionate increase, we can certainly say that a 'gain' in flow in the head will tend, depending on the rest of the setup, lead to a gain in torque/power. If we do a marvellous cylinder head job but fit a camshaft with a weak lift integral (small lift vs degree characteristic, discussed later in Part 6).

I say 'gain' in inverted commas, because a flowbench gain is not an easy thing to describe. There is no such thing as a gain of '20 cfm' (cu ft per minute mass flow) or 12%, except in terms of the bare port flow with no valve before and after, or as a mean value of flow measured at various valve lift points (see flow graph). Are we talking about increase in (1) peak valve lift flow? (2) Flow with no valve? (3) Flow throughout the whole valve lift regime? Because they are all markedly different and can be achieved in different ways, sometimes one at the expense of another.
The true answer is a mix of (1) and (3). The mass flow with no valve (bare port flow, or bpf) will tell us the ultimate flow that the head could generate if no valve were fitted, but the valve represents an intrusion (blockage) to the airstream, always, and it has to be lifted, with some heads, very high to present no obstacle to flow. With luck - and very careful work - we can get the flow under the influence of valve lift to intersect with bare port flow at maximum cam lift -but not often. Usually, depending on the discharge coefficient of the whole valve seat combination, the valve has to be lifted way beyond any achieveable cam lift to present no intrusion, and this is a fact of life with most production heads.

The 'discharge coefficient' of a valve varies with diameter and shape, and whilst bigger valves will flow more air smaller valves are superior to big ones in terms of the absolute ratio of intrusion against flow; in other words multiple small valves will always give a better flow characteristic at all valve lift points (throughout the cam lift regime) than one big one even of equivalent valve head area.

The circuit race cam chosen by the owner in this case offers 10.4mm actual inlet valve lift so there is no point in modifying the head to produce fantastic flow over this valve lift level, the gains have to be concentrated in the 0-10.4mm lift range, and it is possible to achieve this. It is important to note that whilst the valve is at peak lift once during the cycle, it flows air on both opening and closing flanks of the cam.

Of course power does not just come from airflow improvements, cam choice, exhaust header all play a huge part, and here it is worth stressing that whilst the inlet manifold will generate flow losses due to the 90 degree bend between the port axis and the carburettor axis, this loss will be massively outweighed by the avoidance of mixing of the beneficial pressure waves in the ports on each cycle with split carburettor barrels.

Attachments

One of the first flowtests is to determine what the bare inlet port and manifold flow (separately and together), no valves. The IDF manifold will always give a loss over a sidedraft, because of the bend.

Closeup of inlet manifold on head, clay on the intake stops sharp-edge entry loss. This manifold easily outflowed the head - by some 20cfm, so I know straightaway the problem here is first the head, not the manifold.

An F1 head has all these things, but with a production head we are fighting to improve most of them, especially the losses from line of sight (very little in some cases) and the poor short-side radius flow regime most heads have.

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The flow exiting the valve is what counts. Even if the port is capable of huge flow - if the valve and seat combo don't flow, final results will be poor, except at very high valve lift.

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How effective will an alteration be to the short side radius? This 1600 head has big ports, roughly 36-35mm approx, so I'm not going to go much bigger. You need experience to make those judgements.

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I tested several valve shapes after each mod. Left - early 'tulip' valve was dropped with the 131/132 series, doesn't flow well. Center, later 41.7mm valve, surprisingly good, right GC radiused one showed up best.

If the breakaway/transition point (stable to turbulent) is too far from the ssr it causes disruption to the whole airstream. Especially at higher lift it the air will not flow round the ssr, all we can do is minimise the turbulence.

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Stable velocity profile in a smooth constant section port, even minor intrusions to the profile cause a local disruption and flow is upset.

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Airflow only likes straight lines, it drops when there is ANY turn or change of section, a convergence isn't as bad as convergence..

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After each port/seat mod during 40 tests I used up to 5 different valves to find the best 'new' combination. Gradually a pattern emerges.

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Very carefully, because the metal available wasn't much, I induced a burr-finish radius onto the short side region to see what would happen, gain followed..

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Next job is to find out if cutting the seat out to near true valve dia and my normal 70 deg throat helped the flow regime with valve, it did. I roughed out with Neway.

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Next, the ssr is carefully polished at 80 grit, imperative not to be tempted to just grind away the curvature at left and right of the ssr, their transition points are CRITICAL. That work must be done step by step.